We have a brick on concrete front walk. The bricks were mortared onto the concrete and the joints had mortar brushed into them and sprayed with water. They have held up fairly well for many years . We did redo them once the same way and some of the large cracks were mortared by hand. Now many of the joints are crumbling and I wonder if I should so all the joints by hand with mortar. Any suggestions greatly appreciated!

That is going to be a constant maintenance issue. The bricks will always absorb water, and need to move. The grout does not always move as the bricks do, so you get cracks.
You could repair it, and then seal it. Your location is also frost damage prone.
Unless someone has seen something for the northern climates, sealing it and hope for the best is all I have as a solution.
Good luck and tell us how it works out...

Thank you all for your replies. Any suggestions as to what to repoint with and what sealant products containing silicone siloxane would be greatly appreciated. It looks like I have a big job ahead of me.

prosoco or sonneborn but there're many other brands avail,,, make certain you remove ALL the bad mortar & pressure wash to clean sufficiantly,,, then, if you wish, get mortar-in-a-bag,,, we also use mason's socks ( think cake decorating ) & pointing tools.